1. Field of the Invention
This invention relates to a method for manufacturing an semiconductor device and the resulting device. In particular, this invention relates to the structure and process of alignment marks to be used in mixed lithography systems for making semiconductor devices.
2. Related Art
Within the prior art lithography schemes are employed to produce a wide variety of devices. The most common are optical electron beam and X-ray. All have unique attributes and disadvantages. The art has proposed the mixing of these schemes to take advantage of the efficiencies and accuracy offered by them in various stages of device manufacturing. For example U.S. Pat. No 4,612,274 describes a method for using a combination of electron beam (E-beam) and photolithography in the manufacture of an acoustic wave device. The E-beam is used for purposes of writing fine lines which are required in an acoustic wave device while optical lithography is employed to delineate the large scale pad areas.
IBM Technical Disclosure Bulletin, Volume 1, No. 8, pp 3176-3177 (January 1979) relates to a combined E-beam and X-ray lithography process in the manufacture of semiconductor devices. This prior art recognizes that E-beam provides unique alignment capabilities but, at the same time, notes the inherent problems which exist when mixed lithography process are used in terms of maintaining proper registration alignment. The process employed in this TDB employs an alignment mark which is formed in the semiconductor wafer with that alignment mark maintained visible for E-beam registration by evaporating through metal topography which is used as a X-ray absorber for subsequent processing or maintaining rought alignment by means of visible topography in the X-ray absorber layer. As recognized however some comprise in the E-beam alignment accuracy results when relying on visible topography.
Other techniques for providing a registration mark for use with E-beam systems is disclosed in U.S. Pat. No. 3,710,101 and in IBM TDB, Volume 27, No. 1B, pp 686-688 (June 1984). In any scheme employing mixed lithography it is desirable to use the electron beam system for achieving maximum resolution on the critical device levels. The electron beam system is capable of achieving dimensional control that reaches 0.1 .mu.m. In contrast, optical lithography begins to reach its limit at between 0.75 to 0.50 .mu.m and requires a very small process window. Along with dimensional control, electron beam lithography also has the added advantage of superior overlay capability. To fully take advantage of the electron beam system, it is important that the critical device levels that will be written by the electron beam system, be aligned directly to the optically exposed isolation (ROX) level.
In the case of typical optical/E-beam mixed lithography processes, it is common to use a so-called "common zero level" alignment approach. In this technique, once the zero level has been established subsequent levels are written by both the optical and E-beam systems aligned to that zero level. This technique is limited by the fact that all levels are registered using second order alignment. A second order alignment is defined as the registration of one level to another indirectly aligning to a common level. Second order alignments have associated with them a higher degree of overlay error due, in part, to the summation of errors which exist for each of the two levels.
When employing a common zero level process the overlay is not only a second order alignment which, as indicated, is itself a problem, but is also limited by the alignment of the optical system. Stated differently the alignment which is achieved cannot be better than the alignment of the optical system to the common zero level. As such, even when using E-beam systems for purposes of a portion of the fabrication process, the level of registration is inherently limited to that which is achieved by the optical alignment. Thus, within the prior art a need exists to improve overlay accuracy when mathcing an E-beam lithography system to an optical lithography tool.